Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net Income	$ 1,107.3	$ 1,336.0	$ 1,545.3
Other Comprehensive Income (Loss) (Net of Tax and Reclassifications)
Net Unrealized Gains (Losses) on Available for Sale Debt Securities	443.7	(1,474.7)	(534.7)
Net Unrealized Gains (Losses) on Cash Flow Hedges	(0.4)	3.6	0.8
Net Foreign Currency Adjustments	39.0	9.4	10.5
Net Pension and Other Postretirement Benefit Adjustments	(51.0)	(71.9)	59.8
Net Change	431.3	(1,533.6)	(463.6)
Comprehensive Income (Loss)	$ 1,538.6	$ (197.6)	$ 1,081.7